Boilers of thermal power plants have been operated under conditions of markedly high temperature and high pressure in recent years. Part of them are planned to be operated at 566.degree. C. and 316 bar. It is estimated that some of them will operate at 649.degree. C. and 352 bar in the future. Accordingly, materials for such boilers will be used under extremely harsh conditions.
When the operation temperature exceeds 550.degree. C., materials used in the boilers will be changed, for example, from ferritic 21/4% Cr-1% Mo steel to an austenitic steel of high grade such as 18-8 stainless steel in view of oxidation resistance and high temperature strength. Thus, materials of very high grade and high cost are currently used.
Steel materials having an intermediate grade between 21/4% Cr-1% Mo steel and austenitic stainless steel have been searched for in the past several decades. Boiler tube steels containing an intermediate amount of Cr such as 9% Cr steel or 12% Cr steel have been developed on the basis of the demands described above. Some of the steels have attained a high temperature strength and a creep strength comparable to austenitic steels by precipitation strengthening or solid solution strengthening effected by adding a variety of alloying elements as base material components.
The creep strength of heat-resisting steels is governed by solid solution strengthening when the steels have been aged for a short period of time and by precipitation strengthening when they have been aged over a long period of time. This is because solid solution strengthening elements dissolved in the steels are precipitated at first as stable carbides such as M.sub.23 C.sub.6 by aging in many cases. However, when the steels are aged for a still longer period of time, the precipitates are coalescence coarsened, and as a result the creep strength is lowered. Many studies have, therefore, been performed on maintaining the solid solution strengthening elements in a solution state in the steels over a long period of time without precipitation in order to maintain the high creep strength of the heat-resisting steels.
For example, Japanese Patent Unexamined Publication (Kokai) Nos. 63-89644, 61-231139 and 62-297435 disclose ferritic heat-resisting steels which can achieve a creep strength far higher than a conventional Mo-added type ferritic heat-resisting steel by the use of W as a solid solution strengthening element. Many of these steels have a tempered martensite single phase as their structure, and are expected to become the next generation of materials for use in high temperature and high pressure environments due to their advantage as ferritic steels excellent in steam oxidation resistance and due to their high strength properties.
On the other hand, ferritic heat-resisting materials utilize the high strength of a martensite structure containing a large amount of dislocations or its tempered structure formed by the supercooling phenomenon of phase transformation from an austenite single phase region to (ferrite+carbide precipitate) the phase to be produced as a result of cooling during heat treatment. Accordingly, when the structure is subjected to a heat cycle of being reheated to the austenite single region, for example, when the structure is subjected to weld heat affection, the dislocations of high density are relieved again, and the strength is sometimes locally decreased in the weld HAZ (heat-affected zone).
Particularly among those portions which are reheated to a temperature of at least a ferrite-austenite transformation point, portions which has been heated to a temperature near the transformation point, for example, about from 900.degree. to 1,000.degree. C. in the case of 9% Cr steel, and recooled in a short period of time are subjected to martensite transformation while austenite grains do not grow sufficiently to become a fine grain structure. In addition, M.sub.23 C.sub.6 type carbides which are a principal factor in improving the materials strength by precipitation strengthening do not redissolve, and mechanisms for inducing a decrease in the high temperature strength such as alteration of the constituent components of the carbides, or carbide coarsening, may compositely act on the portion to locally become a softened zone. The softening zone-forming phenomenon is termed "HAZ-softening" for convenience.